Re: [RFC PATCH 0/7] tracing/probes: Add more typecast features

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi,

On Mon,  8 Jun 2026 23:24:16 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> Hi,
> 
> Here is a series of patches to introduce more typecast features
> to probe events, which includes 1. expanding BTF typecast to
> fprobe and kprobe events, 2. introducing container_of like typecst
> option, 3. supporting nested typecast, 4. adding $current special
> variable support, 5. adding per-cpu dereference support, 6. adding
> a testcase to check typecasts.

Sashiko found many issues on this series. I'll fix those.

BTW, for $current, I think it should typecasted to task_struct without
typecast. (except if it is the container_of() type typecasting)
In this case, ctx->strcut_btf will be updated automatically if $current
is specified.

Also, I'm thinking redesign +CPU/+PCPU/this_cpu_ptr().
Instead of those, what about introducing followings?

 - this_cpu_read(VAR)
 - this_cpu_ptr(VAR)

Comments are welcome!

Thanks,

> 
> Steve introduced BTF typecast feature for eprobe[1].
> This series extends it and add more options:
> 
> 1. Expanding BTF typecast to kprobe and fprobe.
>    (currently only function entry/exit)
> 
> 2. Introduce container_of like typecast. This adds a "assigned
>    member" option to the typecast.
> 
>    (STRUCT,MEMBER)VAR->ANOTHER_MEMBER
> 
>    This casts VAR to STRUCT type but the VAR is as the address
>    of STRUCT.MEMBER. In C, it is:
> 
>    container_of(VAR, STRUCT, MEMBER)->ANOTHER_MEMBER
> 
> 3. Support nested typecast, e.g.
> 
>    (STRUCT)((STRUCT2)VAR->MEMBER2)->MEMBER
> 
>    the nest level must be smaller than 3.
> 
> 4. Add $current variable to point "current" task_struct.
>    This is useful with typecast, e.g.
> 
>    (task_struct)$current->pid
> 
> 5. per-cpu dereference support.
> 
>    +CPU(VAR) is the same as this_cpu_read(VAR), and
>    +PCPU(VAR) is the same as this_cpu_ptr(VAR).
>    Also, "this_cpu_ptr(VAR)" is available. This is good
>    with nesting expression.
> 
>    (STRUCT)(this_cpu_ptr(VAR))->MEMBER
> 
>    (However, it might be better to allow a special way to omit
>     parentheses for thi_cpu_ptr())
> 
> And added a test script to test part of them.
> 
> [1] https://lore.kernel.org/all/20260601130746.2139d926@gandalf.local.home/
> 
> 
> ---
> 
> Masami Hiramatsu (Google) (7):
>       tracing/probes: Support typecast for various probe events
>       tracing/probes: Support nested typecast
>       tracing/probes: Support field specifier option for typecast
>       tracing/probes: Add $current variable support
>       tracing/probes: Add +CPU() and +PCPU() dereference method to fetcharg
>       tracing/probes: Support reserved this_cpu_ptr() method
>       tracing/probes: Add a new testcase for BTF typecasts
> 
> 
>  Documentation/trace/eprobetrace.rst                |   11 +
>  Documentation/trace/fprobetrace.rst                |   11 +
>  Documentation/trace/kprobetrace.rst                |   12 +
>  kernel/trace/trace.c                               |    6 
>  kernel/trace/trace_probe.c                         |  312 +++++++++++++++-----
>  kernel/trace/trace_probe.h                         |   12 +
>  kernel/trace/trace_probe_tmpl.h                    |   33 ++
>  samples/trace_events/trace-events-sample.c         |   38 ++
>  samples/trace_events/trace-events-sample.h         |   34 ++
>  .../ftrace/test.d/dynevent/btf_probe_event.tc      |   52 +++
>  10 files changed, 422 insertions(+), 99 deletions(-)
>  create mode 100644 tools/testing/selftests/ftrace/test.d/dynevent/btf_probe_event.tc
> 
> --
> Signature


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>